Earphone antennas that are integrally formed to serve both as earphones/headphones and antennas have been used for portable radio terminal devices, portable TV receivers, and wireless communication terminal devices having radio and/or TV functions.
For example, Patent Document 1 (Japanese Unexamined Patent Publication No. 2005-64742 (Tokukai 2005-64742; published on Mar. 10, 2005) discloses a structure illustrated in FIG. 15 that is one of the above earphone antennas. This earphone antenna has a sleeve antenna structure in which a coaxial cable 926 is extended from a wireless communication terminal device, a central conductor (inner conductor) 924 of the coaxial cable 926 is extended from an end of a shield line (outer conductor) 925, and a sleeve antenna is excited by the extended central conductor 924 and the shield line 925.
With the structure, an earphone antenna 900 in which an audio signal and a high-frequency signal received by an antenna are overlapped with each other is obtained.
In the earphone antenna 900 having the above structure, one cable serves as both an audio signal cable and an antenna cable.
Specifically, the central conductor 924 of the coaxial cable 926 is directly connected with a signal line 921a that is one signal line of an earphone 910. Further, the central conductor 924 is connected with a signal line 921b that is the other signal line of the earphone 910, via a capacitor 960 that has a high impedance at a frequency band for an audio signal and has a low impedance at a frequency band for a high-frequency signal.
Further, the shield line 925 of the coaxial cable 926 is connected with the signal line 921b that is the other signal line of the earphone 910, via a high-frequency choke 916 that has a low impedance at a frequency band for an audio signal and has a high impedance at a frequency band for a high-frequency signal.
In the earphone antenna having the above structure, a capacitor and a high-frequency choke coil separate an audio signal from a high-frequency signal received by an antenna.
Because of recent speeding-up of digital circuits, noises from wireless communication terminal devices, such as clocks of CPUs and digital circuits, range from a low frequency to a high frequency. Specifically, the frequency range of noises is very wide, ranging from several 100 kHz to several GHz. These are noises to a received radio wave etc.
Noises directly emitted from the wireless communication terminal devices affect cables of antennas or affect the wireless communication terminal devices via audio signal cables.
Separating signals having noise components by use of capacitors and choke coils as described above is insufficient for preventing the affect of noises. The above conventional earphone antenna has a problem that it is greatly affected by electromagnetic wave noises from the wireless communication terminal device and as a result reception sensitivity of the antenna greatly drops.
The present invention was made to solve the foregoing problems. An object of the present invention is to provide a technique that allows more effectively avoiding the affect of electromagnetic wave noises from a wireless communication terminal device in an earphone antenna device (alternatively, an antenna device having a similar structure) compared with a conventional technique of using only a capacitor or a choke coil, thereby effectively increasing reception sensitivity of an antenna.